Method of electroplating zinc



Dec. 4, 1951 R. F. MAIN ET AL METHOD OF EILECTROPLATING ZINC Filed June 19, 1948 m ma l 1 mm mm M mw Patented Dec. 4, 1951 METHOD OF ELE'CTROPLATING ZINC Ray F. Main and Harold F. Hansen, Chicago, n1., assignors to Acme Steel Company, Chicago, 11]., a corporation of Illinois Application June 19, 1948, Serial No. 34,128

8 Claims. (Cl. 204-55) This invention relates to improvements in electrolytes and in methods of electroplating with the use of a sulfate bath containing zinc salts whereby the coating is lighter in color and has a smoother finish than such a coating applied by the methods heretofore in use.

In the practice of the present commercial methods of electroplating zinc from such sulfate acid baths, difficulty is encountered in obtaining the desired whiteness of the deposit and in obtaining a complete coverage of the base metal, particularly when coatings of small thickness are plated.

v It has been discovered, as a part of the present invention, that the character and quality of the zinc coating is greatly influenced by the presence of lead in the sulfate acid bath. If lead is present, to the extent of five milligrams per litre of the bath, or more, the deposit is substantially darkened in color and is not uniform. Tests made with lead present in the bath in excess of mg. per litre, have produced a product in which the zinc coating is ridged or streaky or carries a visible pattern. If the removal of lead from the bath by deposition is greater than the rate of its introduction, the deposit is dark in color and ridged or streaky or otherwise of poor quality until the lead content has been reduced to substantially 5 mg. per litre. With some types of proprietary electrolytes, best results are not obtained unless the lead content is of the order of 2 mg. per litre or less and, of course, the ideal condition is when the lead content is zero. v

The effect of reducing the quantity of lead present in the bath has been observed using acid solutions of zinc sulphate with or without the presence of buffers or organic. addition agents, The usual types of acid zinc plating solutions have been found to give better results when the lead content is reduced to 5 mg. per litre or less. The electrolyte should preferably contain zinc in the proportion of from 50 to 200 grams per litre with sufiicient sulphuric present to give the electrolyte a pH of from 2.5to 4.0. The temperature of the electrolyte should preferably be between 85 Fahrenheit and 160 F.

The zinc coated product obtained by the present invention is characterized by a whitish color and a uniformity of the appearance and texture of the coating regardless of its thickness. A further advantage of ,the present invention is that, with a minimum of lead in the electrolyte, the usual loose spongy deposits of lead on'the anodes are not present and cleaner anodes are maintained, thereby contributing to the efficiency of the operation. I

It has been found that the results obtained with the present invention are improved by agitation of the bath or by relative movement of the electrolyte and the cathode as by moving it through the bath or rotating it. The improved method of the present invention thus lends itself to the practice of coating in a continuous process in which the strip to be coated is moved continuously with successive portions thereof passing progressively through the coating bath.

In the practice of the present invention, the electrolyte is preferably recirculated continuously through the chamber of the bath, being discharged under pressure ,in the form of jets directed between the electrodes, thereby creating substantial turbulence in the bath with the result that there is a quick replenishment of the solution on the cathodes and higher current densities may be used. For example, the method of the present invention has been practiced with current densities as high as 900 amperes per square foot, in contrast with the usual current densities of from 5 to 250 amperes per square foot.

The following example will illustrate the advantages of the present invention:

Erample A five litre acid zinc electrolyte having a pH of 2.5 to 2.8 was operated for fifteen minutes with a rotating cathode having a surface of 72 square inches area and with a current density of amperes per square foot, at F. At the start, the electrolyte contained 21 mg. of lead per litre. At the end of the fifteen minute period, the cathode was coated with a ridged deposit and was dark gray in color. The lead content at the end of this period was found to be 13.2 mg. per litre. Upon repetition of this plating cycle, of the same duration, using the same portion of electrolyte with the lead content now of 13.2 mg. per litre,'the cathode was again dark gray in color with a ridging and streaking of the deposit which gave it an undesirable appearance. The lead content at the end of this test was 7.9 mg. per litre. A third run under the same condtions and for the same period of time, using the same electrolyte containing 7.9 mg. of lead per litre, at the start, showed a reduction of the ridging, although streaking remained and the coating was grayish in color. At the completion of this test, the elec- {trolyte' contained 4.3 mg. of lead per, litre. A

fourth test starting with this same-portion of electrolyte produced a deposit with'a'slight trace of ridging and streaking and a gray color. The lead content at the completion of this run was 2.2 mg. per litre. A fifth run was made using the same portion of electrolyte and starting with a lead content of 2.2 mg. per litre, under the same conditions and the same duration, resulting in a gray coating with a slightly pitted condition and a trace of streaks. The lead "content at the end of this test was 1.4 mg. per litre. Finally, a sixth run was made under the same conditions and of the same duration whereby there was produced a product having a uniform light gray coating, free of pitting and streaks. These results have been verified by numerous other tests, some of which with other well known electrolytes have demonstrated that the desired surface finish and color are obtained when lead is present to the extent of as much as mg. per litre.

Although the invention is not limited to the use of any particular apparatus it may advantageously be practiced, when treating strip material, with the use of apparatus capable of carrying on the coating operation in a continuous process and of effecting a continuous removal of lead from the electrolyte. Apparatus of that character is illustrated in the accompanying drawin s, in which,

Figure 1 shows a somewhat diagrammatic side elevation of the complete apparatus;

Fig. 2 shows a top plan view of the apparatus illustrated in Fig. 1; and

Fig. 3 shows a vertical section through the purification unit of the apparatus in which a continuous removal of the lead is effected, the section being taken on the line '33 of Fig. 1.

As shown in the accompanying drawings, the apparatus comprises a plurality of units which are supported at two levels by suitable frame structure, not illustrated, so that the metal strip being treated is capable of being moved through one plating bath at one level and returned to the starting point through another bath at another level in the manner disclosed in the United States Letters Patent of Chester M. MacChesney and Ralph H. Norton, No. 1,917,657, dated July 11, 1933. The steel strip IE1, or the like, to be plated, is withdrawn from a supply reel I I, by power applied to the strip beyond the electroplating units, and is carried upwardly in the direction of the arrow l2 and over rollers l3 located at opposite ends of a power washer l4 by which the soil and other loose or rough dirt is removed from the surfaces of the strip. The strip then passes downwardly and under rollers I5 which cause the strip to be immersed in the water or other liquid within the cleaning tank 15. The strip then travels around rollers I! which cause it to pass through the spray tank I8 where it is further cleaned preliminary to passing beneath the rollers IQ of the pickling tank 20 which cause the strip to move through the pickling solution. Upon emerging from the pickling solution the strip l0 passes around roller 22 which cause it to move through another spray tank 23 which removes the adhering picklin solution and prepares the strip for the electroplating operation.

The first stage of the plating operation is carried on in the plating tank 25 in which the metal strip travels beneath the surface of the electrolyte in contact with the rollers 25. The electrolyte is of the type described above and the anodes may be of any standard type and may be formed essentially of high purity zinc and mountedin any approved manner as, for example, in the way electrolyte by the rollers 29. This electrolyte is the same as that in the tank and the anodes in the tank 28 are supported in the same position with respect to the strip as in the first plating tank, so that by reversing the strip in the second tank the proper plating of both sides of the strip is insured.

From the electroplating tank 28, the strip l0 passes back toward its starting position and is carried around aseries of rollers 30 which cause it to be moved through the spray tank 3| in which the adhering electrolyte is removed from the strip. The strip then passes through a drier 33 and thence to the power driven puller 34 where it is extended around a series of rolls 35 before passing to the power driven winding reel 36 by which the coated strip is coiled. The puller 34 supplies the motive power for moving the strip through the preceding units of the apparatus.

Theelectrolyte is contained in a supply tank as and is'continuously circulated and purified for the removal of lead during the operation of the plating process. The electrolyte is circulated by a pump All which withdraws the liquid from the plating tanks 25 and 28 through pipes 42 which discharge into the tank 48 and causes electrolyte to be supplied from the tank to the tanks 25 and 28 through pipes 43. The pipes 43 lead through the purifying units 45 and the filters 46, whereby clean electrolyte substantially free of lead is continuously supplied to the plating baths in the tanks 25 and 28.

In order to create and maintain substantial turbulence of the electrolyte in the plating tanks 25 and 28, eachof these tanks is preferably provided with means for effecting a local circulation of the bath therein. For this purpose, each tank is equipped with a power driven pump having an intake pipe 5| leading from the lower intermediate part of that tank and having a discharge pipe 52 which discharges through a series of spaced nozzles 53 into the same tank. The nozzles are mounted in the sidewall of the tank and are arranged to discharge jets of electrolyte horizontally between the electrodes. By means of these jets the electrolyte is maintained in a stat of substantial turbulence, whereby there is a rapid replenishment of the solution on the cathode and the use of very high current densities is made possible.

The purifying units 45 have the form shown particularly in Fig, 3. Each unit comprises a closed tank 41 having a pipe 43 leading into the side portion thereof at one end and discharging from the side portion thereof at the other end. Within this tank are mounted a plurality of spaced electrodes including the anodes 48 which are mounted on one side wall of the tank and extend horizontally therefrom in parallel relationship. These anodes are formed of insoluble metal and they extend continuously between the top and the bottom walls of the tank with the free edges spacedfrom the opposite side wall of the tank as shown at 49. Arranged in alternate relationship with the anodes 48 are a plurality of cathodes 55 which extend horizontally from the opposite side wall of the tank and between the top and bottom walls thereof, with their lower edges spaced from the wall of the tank to which the anodes are attached, as'shown at 56. The electrolyte flowing throughthe tank is thus compelled to follow a zigzag path and to flow in contact with the opposite faces of each electrode.

'The electrodes 48 and 55 are connected in a direct current circuit and as the electrolyte moves through the tank lead, zinc and copper therein will be plated on the electrodes and therebyremoved. The current density may be so adjusted as to eifect a preferential plating of lead and copper. The cathodes are made readily removable so that they may be replaced when built up to a predetermined extent by deposits of zinc. 1

By the operation of the purifying units 45 during the plating process, it is found that the lead in the electrolyte in the plating tanks 25 and 28 may be continuously maintained below five milligrams per litre and quite uniformly at the preferred operating range of two mg. per litre or less. Thus the desirable effects of the present invention may be obtained in conjunction with" electroplating apparatus should be employed which is substantially free of lead. Other sources of lead are the zinc salts used to make up the electrolyte, the zinc anodes, the acid additions and the various addition agents commonly used.

In the copending application of Allen E.'

Chester, Ray F. Main and Harold F. Hansen, Serial No. 34,129, filed June 19, 1948, a modified form of the present process is disclosed in that certain agents are added to the sulfate acid bath for the purpose of obtaining image bright" zinc deposits upon the cathode, the agents mentioned consisting essentially of a gluconate and an alkylated naphthalene sulfonate.

Although the improved process of the present invention has been explained in connection with the disclosure of a form of apparatus adapted for carrying on the improved method continuously, it will be understood that this invention may be practiced with other forms of apparatus and that it is not limited except as defined by the appended claims.

We claim:

1. The process of electroplating zinc upon a cathode from a sulfate acid solution containing dissolved zinc salts as well as lead as an im-; purity comprising maintaining relative movement between the cathode and the solution, passing an electric current from the solution to the cathode, maintaining the temperature of the solution above 85 F., maintaining the zinc content of the solution above 50 grams per litre, and maintaining the lead content of the solution below 5 milligrams per litre by eleotrodepositing and removing lead as an impurity from the solution independently of the electroplating of the zinc from the solution upon the cathode.

2. The process of electroplating zinc upon a F cathode from a sulfate acid solution containing dissolved zinc salts as well as lead as an im-- purity comprising maintaining relative movement between the cathode and the solution, passing an electric current from the solution to the oath.- ode, maintaining the pH of the solution in the range' 2.5 to'4.0, maintaining the zinc content of the solution above 50 grams per litre, andmaintaining the lead content of the solution below 5 milligrams per litre by electrodepositing and removing lead as an impurity from the solution independently of the electroplating of the zinc from the solution upon the cathode. I

3.'The 'process of electroplating zinc upon a cathode from a sulfate acid solution'containing dissolved zinc salts as well as lead as an impurity comprising maintaining turbulence of the solution adjacent to the cathode, passing an elec-' tric current from the solution to the cathode. maintaining the temperature of the solution above 85 F., maintaining the zinc content of the solution above 50 grams per litre, and maintaining the lead content of the solution below 5 milligrams per litre by electrodepositing and removing lead as an impurity from the solution independently of the electroplating of the zinc from the solution upon the cathode. 1

4. The process set forth in claim 3, wherein the turbulence of the solution adjacent to the cathode is maintained by projecting jets of the solution through the body thereof toward the cathode.

5. The process of electroplating zinc upon an elongated cathode from a sulfate acid solution containing dissolved zinc salts as well as lead as an impurity comprising continuously submerging different portions of the cathode in the solution, passing an electric current from the solution to the submerged portion of the cathode, maintaining the temperature of the solution above 85 F.. maintaining the zinc content of the solution above 50 grams per litre, and maintaining the lead content of the solution below 5 milligrams per litre'by electrodepositing and removing lead as an impurity from the solution independently of the electroplating of the zinc from the solution upon the cathode.

6. The process of electroplating zinc upon a cathode from a sulfate acid solution containing dissolved zinc salts as well as lead as an impurity comprising providing a cell containing the oathode and the solution, continuously moving the cathode through the solution, passing an electric current from the solution to the cathode, maintaining the temperature of the solution above 85 F., maintaining the zinc content of the solution above 50 grams per litre, and maintaining the lead content of the solution below 5 milligrams per litre by continuously circulating the solution from the cell to the exterior and back to the cell and removing lead as an impurity from the circulated solution exterlorly of the cell.

7. The process set forth in claim 6, wherein the circulated solution is electrolyzed exteriorly of the cell in order to remove the lead as an impurity therefrom.

8. The process of electroplating zinc upon a cathode from a sulfate acid solution containing dissolved zinc salts as well as lead as an impurity comprising providing a substantially lead-free cell containing the cathode and the solution, maintaining relative movement between the cathode and the solution and turbulence of the solution adjacent to the cathode, passing an electric current from the solution to the cathode, maintaining the temperature of the solution above 85 F., maintaining the zinc content of the solution above 50 grams per litre, and maintaining the lead content of the solution below 5 milligrams per litre by electrodepositing and removing lead as an impurity from the solution inde- 7 a pez'idehtly o! the blectrbplfiting of the zinc from Hamper, ,Namfi Date thesolution upon thecathode. 2.496;996 Grimth et a1. Feb. 7, 1950 RA? 2,529,700 Lowe et al. v Nov. 14, 1950 HAROLD F. HANSEN. 5 bTHEE REFERENCES REFERENCES CITED mien Property Custodian Pub1ication,'355, 522,

May 18, 1943.

Transactions of the American Electrochemical Society, Vol. 41 (1922), pp. 389-391, 405-409.

The following referehces are of rcort l i1 1 f-h file of this patent:

UNITED STATES PATENTS 10 31am et aL: Principles of Electroplating and Number Name ate Electroforming, 1930, page 319.

1,917,981 L'emetr'e Feb. 20, 1912 'Lange. Handbook of Chemistry, 1934, p. 812. 1,917,657 MacChe'sney et a1. July 11, 1933 Special Volume of the Electrochemical Society,

$471,965 Lowe et a1. May '31, 1949 Modern Electroplating," 1942, pages 345 to 352. 

1. THE PROCESS OF ELECTROPLATING ZINC UPON A CATHODE FROM A SULFATE ACID SOLUTION CONTAINING DISSOLVED ZINC SALTS AS WELL AS LEAD AS AN IMPURITY COMPRISING MAINTAINING RELATIVE MOVEMENT BETWEEN THE CATHODE AND THE SOLUTION, PASSING AN ELECTRIC CURRENT FROM THE SOLUTION TO THE CATHODE, MAINTAINING THE TEMPERATURE OF THE SOLUTION ABOVE 85* F., MAINTAINING THE ZINC CONTENT OF THE SOLUTION ABOVE 50 GRAMS PER LITRE, AND MAINTAINING THE LEAD CONTENT OF THE SOLUTION BELOW 5 MILLIGRAMS PER LITRE BY ELECTRODEPOSITING AND REMOVING LEAD AS AN IMPURITY FROM THE SOLUTION INDEPENDENTLY OF THE ELECTROPLATING OF THE ZINC FROM THE SOLUTION UPON THE CATHODE. 